A fuel cell has been put into practical use, in which fuel supply spaces provided on one side of an electrolyte layer and supplied with gaseous fuel are connected in series.
Further, a flow-type fuel cell has been put into practical use, in which a plurality of fuel supply spaces are provided in a cascade pattern, in which the fuel supply spaces are connected in parallel with the number thereof decreasing gradually toward a downstream side.
According to the cascade pattern, the reduction in a flow rate on the downstream side caused by the consumption of gaseous fuel through the electrolyte membrane is compensated for. As a result, the stable supply flow rate of gaseous fuel in the fuel supply spaces can be realized from the upstream side to the downstream side.
Further, an air breathing type fuel cell has been put into practical use, in which a polymer electrolyte membrane is used as an electrolyte layer, and oxygen in the atmosphere is taken in through atmospheric communication spaces communicated with the atmosphere provided on one side of the polymer electrolyte membrane. As a result, electricity is generated by the electrochemical reaction between gaseous fuel and oxygen.
The polymer electrolyte membrane is not a completely air-tight film. Therefore, when the fuel supply spaces and the atmospheric communication spaces are provided with the polymer electrolyte membrane interposed therebetween, nitrogen in the atmosphere diffuses from the atmospheric communication spaces to the fuel supply spaces due to the concentration.
The nitrogen having entered the fuel supply spaces decreases the power generation efficiency by decreasing the partial pressure of gaseous fuel in the fuel supply spaces. Therefore, it is desirable to purge impurity gas containing nitrogen from the fuel supply spaces to the atmosphere by performing purge periodically.
In the fuel cell system of Japanese Patent Application Laid-Open No. 2004-536438, a dead end type fuel cell is shown, which includes a fuel cell stack, a purge valve, an actuator, a controller, and a sensor.
In the fuel cell stack in the fuel cell system, fuel is introduced in a cascade pattern. Therefore, during the operation of the fuel cell stack, impurities are likely to be accumulated in a purge cell portion disposed on the most downstream side.
When the impurities are accumulated in the purge cell portion, the performance of the purge cell portion degrades and the voltage of the purge cell portion is decreased.
A purge operation is performed using the following structure.
The purge valve is provided on the downstream side of the purge cell portion. In a fuel cell operated in a dead end mode, the purge valve is usually closed.
Impurities accumulated in the purge cell portion are released when the purge valve is opened.
Herein, the actuator is provided so as to open/close the purge valve.
Power is supplied to the sensor, actuator, and controller by the fuel cell stack.
The controller performs a purge operation when the sensor monitors the voltage in the purge cell portion, and the voltage in the purge cell portion decreases.
In the purge operation, the actuator opens the purge valve in response to a control signal from the controller. As a result, the impurities accumulated in the purge cell portion are released.
However, the fuel cell system shown in Japanese Patent Application Laid-Open No. 2004-536438 is not satisfactory in terms of the miniaturization.
More specifically, in the fuel cell system, in the case where the voltage in the purge cell portion is monitored with the sensor and the voltage in the purge cell portion decreases, the actuator opens the purge valve in response to the control signal from the controller. As a result, the purge operation is performed.
Therefore, the sensor for monitoring the voltage in the purge cell portion, the controller for determining purge conditions and controlling the open/close of the purge valve are required. Thus, the above-mentioned fuel cell system is not considered to be suitable for miniaturization.